Field of the Invention
This disclosure relates to a method of manufacturing a member having concentric surfaces including a diffraction optical element or a diffraction optical element forming die used for an optical equipment such as a camera and a video, an optical member having a diffraction grating, and an optical element.
Description of the Related Art
An optical member having a diffraction grating, for example, a diffraction optical element, has characteristics such as having an opposite dispersing property to that of the refraction optical element and contributing to achieve a compact optical system that a refraction optical element including lens and prisms lacks, and hence is used in various optical systems such as optical instruments.
A method of manufacturing an optical member having a diffraction grating is disclosed in Japanese Patent Laid-Open No. 11-197902. Specifically, when forming the shape of a diffraction surface while rotating a diffraction surface to be cut and a cutting tool relatively to each other and translating simultaneously, an inclined surface having a diffracting function is cut by using a side cutting edge having a shorter length than the inclined surface having the diffracting function. Japanese Patent Laid-Open No. 11-197902 also discloses that the cutting tool is attached so that a longitudinal direction of the side cutting edge forms an angle of 30 or smaller with respect to the inclined surface, and the diffraction surface to be cut is relatively moved at an angle of 30 degrees or smaller from a lower side to a higher side of the inclined surface to achieve a cutting process.
Technical Problem
Japanese Patent Laid-Open No. 11-197902 discloses the optical member including a diffraction grating having the same characteristic as a concave lens, specifically, the optical member having a diffraction surface facing inward and an inclination of outer grating and inclined more on the outer side. In a case where this method is applied to an optical member, for example, a plastic lens, by the diffraction grating having the same characteristics as a convex lens (hereinafter, referred to as a projecting grating), there arises the following subject. Here, the optical member on the basis of the projecting grating means a projecting member provided with the diffraction grating having the diffraction surface facing outward and inclined more on the outer side.
In order to process the diffraction surface of the projecting grating from the lower side to the higher side, a distal end of the cutting tool needs to reach the cutting tool to a lowermost point of the diffraction surface of a grating closest to an outer peripheral end portion (hereinafter an outermost grating) while proceeding from the outer peripheral end portion toward a center of the plastic lens. In order to finish a surface roughness from the outer peripheral end portion to the outermost grating efficiently, a gap angle formed between the side cutting edge of the cutting tool and a cross-sectional surface vertical to an axis of rotation needs to be reduced to set a feeding speed as fast as possible. However, if the gap angle is smaller than the inclination of the diffraction surface of the outermost grating, the cutting tool (an opposite side to the distal end of a tool of the side cutting edge) interferes with the diffraction surface of the outermost grating. In order to avoid this interference, increasing the gap angle and lowering the feeding speed of the cutting tool are conceivable. However, since a processing efficiency is lowered and the gap angle is increased so that a crushing effect of the cutting surface by the side cutting edge (hereinafter, referred to as a burnishing effect) is also lowered, there is a case where the surface roughness is deteriorated.
If the outermost grating has a shape failure due to the interference with the tool, an optical performance of the processed plastic lens may be lowered. If there is the shape failure of the outermost grating and the surface roughness from the outer peripheral end portion to the outermost grating is deteriorated, an appearance quality of the optical member is varied so that the appearance quality may be lowered.
This disclosure provides a method of manufacturing a member having a desired projecting grating, an optical member, and an optical element.